Easy answer?? It doesn't sell.

Pragmatic answer? Perhaps it doesn't store or transport as well. It's all well and good if, for example, the US can produce 50% of the world's Miracle Grain #1, but it rots in a week - half the world will never see it.

My best friend's wife has a myriad of health problems (some are probably self-induced, but I digress). Anyway, they live on a variety of odd grains - corn is probably the most normal of they food staples they eat. But it's very hard for them to find non-wheat, non-rice foods. Maybe in downtown LA it wouldn't be so difficult, but up in eastern WA, it's a hassle.